Gas-phase polymerization processes are economical processes for producing polyethylenes. Such processes can, in particular, be carried out in gas-phase fluidized-bed reactors, in which the polymer particles are kept in a fluidized state by means of an appropriate gas stream. However, to allow a stable production of polyethylenes with constant product quality it may be necessary to carry out the polymerization in a way that local overheating in catalyst particles, so-called hot spots, and sheeting at the reactor walls are avoided because such phenomena may result in the formation of lumps or agglomerates. However, the formation of lumps or agglomerates is highly undesirable because the quality of the prepared polymers may be reduced and plugging of the product discharge system with a subsequently required shut-down of the polymerization process may occur.
One possibility to improve the operability of gas-phase polymerization reactors, i.e. to reduce the tendency for forming such lumps or agglomerates, is the manner of how the catalyst is fed into the polymerization reactor and how it is pretreated before being fed. Many possibilities have been described. WO 2010/076289 A1, for example, discloses a process for introducing a catalyst powder based on a titanium compound supported on magnesium halide into a gas-phase olefin polymerization reactor in which the catalyst powder is first contacted with a liquid phase comprising an organo-aluminum compound and optionally an external donor compound and then the activated catalyst powder is introduced in a gas-phase olefin polymerization reactor. However, the operability of the process may still be unsatisfactory for some polymerization conditions.
It has been found that the operability of polymerization processes can be improved if a prepolymerized catalyst is fed to the polymerization reaction. The prepolymerization can be carried out in the course of preparing the polymerization catalyst or immediately thereafter. However, this means that higher amounts of catalysts, in terms of solid powder, have to be processed because prepolymerization increases the mass of the catalyst and may necessitate that activated catalysts have to be stored and transported. Consequently, performing the prepolymerization immediately before introducing the catalyst into the polymerization reactor may be necessary.
EP 560 312 A1 describes a continuous process for the gas-phase polymerization of ethylene and its mixtures with α-olefins, in which the product resulting from contacting a titanium halide supported on an active magnesium dihalide and an aluminum alkyl compound, optionally in the presence of polymerizable olefin, is prepolymerized with ethylene or an ethylene α-olefin mixture in an amount of 30 to 1000 g/g of solid catalyst component and the polymerization of ethylene or its mixtures with α-olefins is carried out in gas-phase in the presence of an alkane having from 3 to 5 carbon atoms using the resulting prepolymerized catalyst system. The steps of preforming and of prepolymerizing the catalyst and the presence of the alkane in the gas phase make it possible to easily control the gas-phase polymerization process. One drawback for such a process is the walls of the prepolymerization reactor may be coated with the polymeric material and a shut-down of the polymerization may be necessary for recovery of the material. Furthermore, the polyethylene products sometimes contain inhomogeneities like specks and, particularly in large commercial plants, the requisite amount of first prepared pre-polymer may necessitate that relatively large equipment be installed ahead of the first gas phase reactor.
Alternative possibilities for carrying out a prepolymerization immediately before entering the catalyst into the polymerization reactor have been investigated. For example, WO 2007/033941 A1 discloses a process for the gas-phase polymerization of olefins in which the polymerization catalyst is prepolymerized with one or more olefins in a gas-phase tubular reactor before being fed to the gas-phase polymerization reactor. However, the gas-phase tubular reactor for carrying out the prepolymerization was very sensitive with respect to its operability. Accordingly it was the object of the present technology to overcome the challenges in the relevant technical field and to find a gas-phase polymerization processes for producing polyethylenes that avoids wall sheeting in the prepolymerization reactor and prevents the formation of lumps or agglomerates in the polymerization reactors, resulting in polyethylene particles having good particle size distribution with a reduced formation of fine polyethylene particles, polyethylenes with a good homogeneity and not requiring large-dimensioned apparatuses for the prepolymerization.